1. Field of the Invention
This invention relates to an apparatus for separating an entrained gas from a liquid. More particularly, a fine mesh screen transports the liquid by capillary action but inhibits the flow of entrained gas. Following gas separation, the liquid may be used for applications requiring a gasless, or reduced gas content, liquid including spacecraft propulsion, chemical process control and medical applications.
2. Description of Related Art
Spacecraft, such as satellites, frequently utilize electrothermal arcjet thrusters for attitude and altitude adjustments. As disclosed in U.S. Pat. No. 4,995,231 to Smith et al., that is incorporated by reference in its entirety herein, electrothermal arcjet thrusters convert electrical energy to thermal energy by heat transfer from an arc discharge to a flowing propellant and from thermal energy to directed kinetic energy by expansion of the heated propellant through a nozzle.
Most electrothermal arcjet thrusters have as common features an anode in the form of a nozzle body and a cathode in the form of a cylindrical rod with a conical tip. The nozzle body has an arc chamber defined by a constrictor in a rearward portion of the body and a nozzle in a forward portion thereof. The cathode rod is aligned on the longitudinal axis of the nozzle body with its conical tip extending into the upstream end of the arc chamber in spaced relation to the constrictor so as to define a gap therebetween.
An electric arc is first initiated between the cathode rod and the anode nozzle body at the entrance to the constrictor. The arc is then forced downstream through the constrictor by pressurized vortex-like flow of a propellant gas introduced into the arc chamber about the cathode rod. The arc stabilizes and attaches at the nozzle. The propellant gas is heated in the region of the constrictor and in the region of the arc diffusion at the mouth of the nozzle downstream of the exit from the constrictor. The superheated gas is then exhausted out the nozzle to achieve thrust.
The gaseous propellant for an electrothermal arcjet thruster is typically formed by catalytic decomposition of a liquid propellant in a gas generator. One liquid propellant employed for spacecraft propulsion is an autocatalytic liquid (spontaneously decomposes to gaseous products on contact with a catalyst), such as hydrazine (N2H4). Hydrazine decomposes to hydrogen gas and nitrogen gas on contact with an iridium catalyst in a gas generator. The hydrogen and nitrogen gases are converted to a high temperature plasma in an arcjet thruster and expelled at supersonic speeds through a nozzle propelling the spacecraft.
Typically, the liquid propellant is stored in a fuel tank. A pressurized gas pressurizes the liquid propellant so that the opening of a downstream thrust control valve initiates flow of the hypergolic fluid to the gas generator. The use of a pressurized gas to deliver a monopropellant to a gas generator is disclosed in U.S. Pat. No. 5,746,050 to McLean et al., that is incorporated by reference in its entirety herein. Pressurized helium gas is used to displace liquid hydrazine in the fuel tank. Helium bubbles may become entrained in the liquid hydrazine. By entrained, it is meant that the helium bubbles are suspended in the liquid and mechanically transported with the liquid, as distinguished from dissolution. Helium may also be dissolved and go into solution in the hydrazine. Dissolved helium may reform as entrained bubbles when the liquid hydrazine pressure drops.
At typical pressures employed in spacecraft propulsion systems, on the order of 80 psia to 100 psia at the gas generator, the helium bubbles can be large enough to cause the liquid hydrazine to be delivered to the gas generator as liquid slugs separated by helium bubbles or as liquid interspersed with a plurality of bubbles. When the liquid hydrazine contacts the catalyst in the gas generator, it rapidly decomposes into a large volume of gas, increasing the pressure in the gas generator. Helium does not react with the catalyst in the gas generator so that when a helium bubble passes through the gas generator, there is no increase in gas volume and the pressure in the gas generator drops. Fluctuations in the gas generator between high pressure when hydrazine is present and low pressure when helium is present causes pressure and mass flow rate oscillations in the gaseous product output. In an arcjet thruster, these pressure and mass flow rate oscillations may cause the electric arc to travel from the nozzle back into the throat generating the potential for the electric arc to either extinguish or erode the throat.
Spacecraft propulsion thrusters are generally small thrusters used after the spacecraft approaches orbital elevation and is in a microgravity environment. The spacecraft propulsion thrusters are used to make altitude and attitude adjustments to place, or return, the spacecraft in a precisely desired location. The adjustments require a continuous flow of liquid propellant for a precise amount of time. Gas entrained in the flow can lead to undesirable interruptions in the physical or chemical process taking place in the thruster. The flow interruptions can lead to instabilities or damage and also significantly reduce the lifetime of the unit.
Among the detrimental effects of gas entrained in the liquid propellant are positive voltage excursions whereby gas entrained in the liquid propellant at a pressure of 255 psia expands when transiting a fluid resistor and exiting the fluid resistor at a pressure of 100 psia. Rapid expansion of the gas provides a surge in the propellant flow downstream of the expanding gas increasing the volume of propellant gas delivered to the arcjet thruster and causing a voltage rise. A bubble of the size 0.040 cm3 (at 255 psia) into the fluid resistor can cause a high voltage shutdown of the arcjet thruster.
Negative voltage excursions can occur when helium gas passes into the gas generator displacing hydrazine. This reduces the flow of propellant gas to the arcjet thruster causing the arc to pull back toward the constrictor. The increased concentration of arc on the electrode surface at the constrictor can cause abnormal heating and possible damage. Repeated gas ingestion increases electrode wear and possibly reduces thruster operating life.
Another scenario resulting in a negative voltage excursion is that the entrained gas is either generated or collected at the thrust control valve during arcjet operation and eventually migrates to the gas generator as a large gas bubble starving the gas generator of hydrazine and reducing the flow of propellant. Tests and analytical results indicate that a bubble as small as 0.001 cm3 (at 255 psia) can cause a measurable voltage drop-off at the arcjet thruster, on the order of 5 volts. A bubble size of 0.008 cm3 (at 255 psia) can cause a low voltage shutdown of the arcjet thruster.
Within a fuel tank, it is known to use propellant management devices to employ capillary action to draw liquid propellant from the tank while inhibiting the flow of a gaseous pressurant as disclosed in U.S. Pat. No. 4, 272,257. One material disclosed in U.S. Pat. No. 4,743,278 for use in a propellant management device is a titanium or steel sheet with small, on the order of 0.0015 inch, perforated holes. A description of the use of capillary action to separate a gas from a liquid is found in U.S. Pat. No. 5,711,877.
Each of U.S. Pat. No. 4,272,257; 4,743,278 and 5,711,877 is incorporated by reference in the entirety herein.
U.S. Pat. No. 5,746,050 discloses devices such as ultrasonic transducers and externally driven flow agitators disclosed upstream of the gas generator to reduce the size of gas bubbles entrained in the liquid propellant and thereby reduce the potential for damage to the gas generator and the arcjet thruster. The propellant management devices disclose methods for reducing or eliminating entrained gas from liquid propellant exiting a fuel tank but do not address problems associated with entrained gas downstream of the fuel tank. There remains, however, a need for a liquid/gas separator that removes entrained gas as opposed to the reduction of bubble size and is capable of removing the entrained gas downstream from the fuel tank.
Accordingly, it is a first object of the invention to provide a gas arrestor that effectively separates an entrained gas from a liquid medium. It is a second object of the invention to incorporate the gas arrestor into a spacecraft propulsion system such that entrained gas does not interfere with proper operation of spacecraft thrusters.
It is a feature of the invention that the gas arrestor is a porous member having pores of a size effective to enable the passage of liquid medium while inhibiting the transport of the gas due to the surface tension of the gas. It is another feature of the invention that the porous member is preferably a woven screen mesh and may be formed from titanium or a titanium base alloy. It is a feature of one embodiment of the invention that a wick extends from the porous member in an upstream direction to prevent the formation of a liquid flow blocking gas bubble. Still another feature of the invention is that the gas arrestor may be positioned downstream of the thrust control valve. An advantage of this feature is that when the thrust control valve is turned off, trapped gas and remaining liquid is vented through the gas generator and reaction chamber into space. This provides passive and cyclic removal of the gas without the requirement of an additional vent element.
Among the advantages of the invention are that a gas free, or substantially gas free liquid is provided for use in spacecraft applications or other applications requiring such a liquid. By positioning the gas arrestor downstream of a liquid fluid resistor, that reduces the pressure of the liquid medium, dissolved gas that generates bubbles on the pressure reduction is trapped by the gas arrestor.
In accordance with a first embodiment of the invention, there is provided a gas arrestor for separating an entrained gas from a liquid. The gas arrestor includes an inlet coupling a source of the liquid having an entrained gas component from a reservoir contained within the gas arrestor. A wall of the reservoir is formed from a first side off a porous member. The porous member has the first side and an opposing second side with pores extending therebetween. The pores are of a size effective to enable the flow of the liquid from the first side to the second side while inhibiting the flow of the entrained gas component. In one embodiment, a wick extends from the first side in a direction toward the inlet and a second conduit that is adjacent the second side receives the liquid following entrained gas separation.
In accordance with a second embodiment of the invention, there is provided a system for the delivery of a gaseous product to a reaction chamber. This system includes a fuel tank containing a liquid propellant, a pressurized gas communicating with the fuel tank and a thrust control valve disposed between the fuel tank and the reaction chamber. A gas arrestor is disposed between the fuel tank and the gas generator. The gas arrestor is effective to remove gas entrained in the liquid propellant. A gas generator receives the liquid propellant from the gas arrestor effecting decomposition of the liquid propellant to reaction gases and delivers the reaction gases to the reaction chamber.